1. Field of the Invention
The present invention relates to controlling device for DC motor speed variation, particularly, to a controlling device for DC motor speed variation which can alleviate shock arising from starting and stopping of a DC motor to protect it from rapturing by sudden variation of the armature current, and also able to interlock direction of motor rotation, at the same time, it is able to provide overcurrent protection for the DC motor.
2. Description of the Prior Art
FIG. 1 is a control circuit diagram for a conventional controlling device for DC motor speed variation. As shown in FIG. 1, the (+) input terminal of a DC motor 16 is connected to two relays 11 and 13, whereas its (xe2x88x92) input terminal is connected to relays 12 and 14. The normally open points of the relays 11 and 12 are both connected to a contactor 2 which is further connected to the (+) terminal of a power supply circuit 1. On the other hand, a common point of the relays 13 and 14 is connected to another contactor 6 which is further connected to the (xe2x88x92) terminal of the power supply circuit 1. All four relays 11, 12, 13, 14 are controlled by a common control circuit 15 for operation, Meanwhile, the contact points of the relays 11, 12, 13, 14 are opened when they are de-energized.
In the above described conventional circuit, if it is intended to drive the DC motor 16 to rotate in normal direction, the control circuit 15 energizes the relays 11 and 14 to close their contact points thereby a current flows from the (+) terminal of the power supply circuit 1 to the (xe2x88x92) terminal thereof by way of the contactor 2, and the initially normally open point (now closed) of the relay 11, (+) terminal and (xe2x88x92) terminal of the DC motor 16, the relay 14, and the contactor 6 therefore forming a complete electrical circuit for the current to circulate so as to enable DC motor 16 to rotate in normal direction. If it is intended to stop the DC motor 16, the control circuit 15 actuates the relays 11 and 14 to restore their initial state by opening their contact points so as to interrupt the power supply to the DC motor 16. However, the DC motor 16 can not stop instantly owing to its inertia. Besides, if it is intended to drive the DC motor 16 in reversed direction, the control circuit 15 energizes the relays 12 and 13 to close their contact points, at this time the current flows from the (+) terminal of the power supply circuit 1 to the (xe2x88x92) terminal of the circuit 1 thereof, by way of the contactor 2, the initially normally open point (now closed) of the relay 12, (xe2x88x92) terminal of the DC motor 16, the (+) terminal of the DC motor 16, the relay 13, and the contactor 6 therefore forming a complete electrical circuit for the current to circulate so as to enable the DC motor 16 to rotate in reversed direction.
Owing to the fact that the rotational direction of the DC motor 16 is controlled by only on/off operation of the relays 11, 12, 13 and 14, instantaneous on/off operation of them for starting or stopping the DC motor 16 is sure to irritate a heavy sparking at their contact points by an transient current produced in the circuit resulting in shortening the lifetime of the relays 11, 12, 13 and 14. Further to this, when the DC motor 16 suddenly ceases rotating, the sparking at the open contact points of the relays caused by an induced back emf of the DC motor 16 will likely ruin the contact points. Besides, there is no provision of overload protection means for this system, nor provision of device for constant supervision of current during operation of the DC motor 16 resulting in lack of electric security.
Aiming at the above depicted shortcomings, the present invention it to propose a newly developed controlling device for DC motor speed variation capable of operating the DC motor securely.
It is an object of the present invention to provide controlling device for DC motor speed variation which can alleviate shock arising from starting and stopping of a DC motor to protect it from rapturing by sudden variation of the armature current.
It is another object of the present invention to provide controlling device for DC motor speed variation which can offer interlocking means for motor rotational direction.
It is still another object of the present invention that this control device is able to provide overcurrent protection for the DC motor.
To achieve these and other objects mentioned above, the controlling device for DC motor speed variation essentially comprises a power supply circuit, two relays, and an interlock circuit. Wherein the power supply circuit is for providing a DC power source; each of the two relays includes an exciting coil, a common point, a normally closed point, and a normally open point, before energizing the exciting coil, the common point is communicated with the normally closed point, but is not communicated with the normally open point, after the exciting coil is energized, the common point is communicated with the normally open point, but is not communicated with the normally closed point. The interlock circuit includes two output terminals each of them being connected to the exciting coil of one of the two relays respectively, the exciting coil is energized when the output terminal is energized, but the two output terminals are never energized simultaneously. The two input terminals of the DC motor are respectively connected to the common points of both relays, and the two normally closed points of both relays are connected to one terminal of the power supply circuit, whereas the two normally open points of both relays are connected to the other terminal of the power supply circuit.
Further to this, an electronic switch is connected to the power supply circuit, this electronic switch is actuated by a starting control circuit capable of delaying actuating time for the electronic switch. Besides, a current detecting circuit is provided for the power supply circuit for detecting whether the motor is over loaded.